This invention relates to a process for the concentration of dilute sulfuric acid solutions, particularly dilute solutions containing iron (II) sulfate and other metal salts obtained, for example, during the production of titanium dioxide by the xe2x80x9csulphatexe2x80x9d process.
In the xe2x80x9csulphatexe2x80x9d process, the raw material (ilmenite or slag) is ground and dissolved in concentrated sulphuric acid to give a solution of titanium sulphate. This solution is hydrolysed to convert the titanium sulphate to titanium dioxide which is filtered off. The solution remaining after filtration contains 10-30% by weight sulphuric acid as well as large amounts of iron (II) sulfate and other metal salts. In the following description, this sulfuric acid is designated as xe2x80x9cwaste acidxe2x80x9d.
In order to be able to reuse the sulfuric acid in the waste acid, and particularly to be able to recycle it to the titanium dioxide manufacturing process, it is necessary to reconcentrate the waste acid and to separate the metal salts therefrom to the greatest possible extent. In the various known processes, the waste acid is reconcentrated to a concentration of 60 to 70% by weight sulphuric acid, the precipitated metal salts are separated out, and the reconcentrated acid is then either directly reused or concentrated further.
As storing the waste acid is impractical, any factor which affects the efficiency of the concentration process, particularly if it results in the process having to be halted, also has an effect on the rate of titanium dioxide production.
EP-A425000 discloses a process for concentrating dilute sulphuric acid using a multistage system, each stage consisting of a heat exchanger and a vacuum evaporator. The heating medium used is steam and to increase the economics of the process, the steam generated by the evaporation step in the second stage is used as the heating medium in the first stage.
It is well known that one of the problems associated with such a process using heat exchangers is that as the concentration of the acid increases, there is an increasing tendency for the salts to come out of solution and be deposited on the surface of the heat exchanger, resulting in the process having to be halted whilst such deposits are removed.
Although the major component of the metal salts present is ferrous sulphate, deposition of this material does not produce a particularly difficult problem as such deposits, being water soluble, are easily removed by washing.
A much greater problem is caused when calcium sulphate is present in the dilute acid as this material is insoluble in water and hence removal of deposits is expensive and time consuming. Moreover, calcium sulphate is a very efficient heat insulator and deposition rapidly affects the efficiency of the process.
The problem of calcium sulphate deposition is worst at acid concentrations between circa 34% and 39%. Below these concentrations the calcium sulphate stays in solution and above them a precipitated salt phase forms (comprising ferrous sulphate as well as calcium sulphate) so there is no precipitation on surfaces directly from solution.
According to EP-A425000, the concentration of the acid in the first stage should be kept at 40% or less, i.e. it is suggesting that the system should be operated in a manner leading to maximum calcium sulphate deposition.
EP-A-425000 also suggests that it may be useful to heat the dilute acid prior to the concentration process using waste heat.
In accordance with common chemical engineering terminology, xe2x80x9cwaste heatxe2x80x9d refers to heat, usually in the form of steam or hot water, which has been used previously as a heating medium and which is consequently at a lower temperature than when originally generated and is to be distinguished from xe2x80x9clivexe2x80x9d heat, again usually in the form of steam or hot water, which has been generated at a temperature suitable for a particular purpose.
A suitable source of such heat is hot water resulting from condensation of the steam used in steam micronisers which are commonly used to mill the titanium dioxide to produce pigment grade material.
A more economic process than described in EP-A-425000 is to use the waste heat in a heat exchanger/evaporation system. In accordance with the disclosure of EP-A425000, the logical position of such a system is as a pre concentrator.
According to one aspect of the invention, there is provided a process for the concentration of dilute sulphuric acid containing metal salts using a multistage heat exchanger vacuum evaporator system, comprising the steps of
a) feeding part of the acid to the first stage of the multistage system operating at an acid concentration of less than 30%.
b) feeding the remainder of the dilute acid to a separate heat exchanger vacuum evaporator using waste heat as the heating medium and operating at an acid concentration of greater than 40%.
c) feeding the acid from steps (a) and (b) to the second stage of the multistage system operating at a concentration of greater than 50% and recycling steam generated in the evaporator as the heating medium for the first stage, and
d) feeding the acid from the second stage to such subsequent steps as are necessary to produce the required final acid concentration.
According to a further aspect of the invention, the process comprises the steps of
a) feeding the dilute acid to the first stage of the multistage system operating at an acid concentration of less than 30%
b) feeding part of the acid from step (a) to a heat exchanger/vacuum evaporator system heated using waste heat and operating at an acid concentration greater than 40%.
c) feeding the acid from step (b) with the remainder of the acid from step (a) to the second stage of the multistage system operating at an acid concentration greater than 50% and recycling steam generated in the evaporator as the heating medium for the first stage, and
d) feeding the acid from the second stage to such subsequent steps as are necessary to produce the required final acid concentration.
In this manner, circulation of acid through a heat exchanger/evaporation stage at a concentration at which there is maximum calcium sulphate deposition, is avoided.
In addition, interposing the stage in which waste heat is used as the heating medium between stages 1 and 2 means that the heat exchange between the steam generated in stage 2 and the relatively cooler acid in stage 1 is more efficient.
The invention will be more readily understood from the following description drawing and example.